This invention finds industrial applicability in measuring x-ray and synchrotron radiation powder diffraction data using one- and two-dimensional detectors such as a position sensitive proportional counter (PSPC), an imaging plate and a large CCD detector.
An essential problem that should be addressed in measuring powder x-ray diffraction data is how to achieve efficient improvement in the angular resolution and the statistical precision of data. One means for solving the problem of statistical precision is using a one- or two-dimensional detector. This not only shortens the time required to measure the whole powder diffraction data; in addition, the time for measurement at an individual point is equal to the time for measuring the whole powder diffraction data and, hence, the overall time of measurement can be shortened by a factor of at least 10, enabling a marked improvement in statistical precision to be realized efficiently.
For improvement of angular resolution, it is effective to perform a parallel-beam x-ray diffraction experiment using a highly directional x-ray source such as synchrotron radiation. On the other hand, the effort to improve the angular resolution of data is limited by the position resolution of the detector, or the size of a detection unit. For example, it is impossible to get information finer than a data detection unit (pixel) in the detector such as an imaging plate. Hence, in order to measure finer (higher-resolution) data, various efforts have been made to reduce the actual size of the detection unit or improve the apparent resolution of the detection unit by increasing the size of a measuring instrument such as a diffractometer on which the detector is to be mounted.
Angular resolution is the resolution of a diffraction angle in x-ray diffraction data and high angular resolution means a small half-peak width of powder x-ray profile (which is the width at half intensity as expressed by diffraction angle). Angular resolution is expressed in degrees.
The imaging plate is a new x-ray film prepared by coating a plastic film with a stimulable phosphor that serves as a memory of x-ray energy. Unlike PSPC and CCD that perform measurement with sensing elements that are fixed in absolute positions on the detector, the imaging plate is capable of detecting x-rays in a desired position on the film (20 cmxc3x9740 cm) and the detection unit can be set in any desired position. In addition, the imaging plate has high sensitivity and wide exposure latitude. Because of these advantages, the imaging plate is currently one of the most commonly used two-dimensional detectors.
The following may be mentioned about the measurement of x-ray diffraction data using the imaging plate.
(1) At the beginning of 1990, the size of a pixel as the detection unit was 100 xcexcm square. Later technological development reduced the pixel size to 50 xcexcm square and even to 25 xcexcm square. However, the resolution of silver halide films is about 10 xcexcm square and it will be many years before this level is achieved by the imaging plate.
(2) The apparent resolution can be improved by increasing the size of the diffractometer on which the imaging plate is to be mounted and thereby increasing the distance of measurement. However, if the distance of measurement is doubled, the resolution can only be increased by a factor of two; in other words, a bigger diffractometer does not achieve an outstanding improvement in resolution.
(3) With the advent of an x-ray source having ultra-high resolution such as synchrotron radiation, there has been a growing demand to improve the resolving power of the detector.
Thus, a new approach has been desired in order to improve the resolution in measurement of x-ray diffraction data using a one- or two-dimensional detector.
A prior art technology for providing improved position resolution in measurement with a one- or two-dimensional detector is a radiation imaging device for typical use in x-ray medical equipment that is designed to obtain two-dimensional image data by displacing a two-dimensional array sensor in both a horizontal and a vertical direction by a distance equal to one half the pixel width (Japanese Patent Laid-Open No. 372892/1992).
Similar prior art techniques include the following: a mask is placed before a two-dimensional detector that is smaller than the detection unit and moved relative to the detector relative so as to provide a fine two-dimensional image (Japanese Patent Laid-Open No. 82305/1994); a two-dimensional detector is moved by small distances in an imaging device in an x-ray interferometer to produce composite imaging data and the detection efficiency of each sensing element is corrected to enhance the space resolution (Japanese Patent Laid-Open No. 51881/1999); and a one-dimensional x-ray detector is moved by small distances to produce a fine two-dimensional image (Japanese Patent Laid-Open No. 10220/1989).
There are two other techniques based on a similar concept. In one approach, a detector array consisting of multiple one-dimensional rows of radiation sensing elements is moved in a predetermined direction as the output data from each element is gathered by a photon counting method so as to capture a two-dimensional radiation image (Japanese Patent Laid-Open No. 204283/1992); the other approach is a radiation CT apparatus which allows an object to be exposed to a radiation such as x-rays or xcex3-rays at a fan beam angle and in which the step of translating the radiation source and the detector relative to each other and the step of rotating the detector about the object are repeated by turns to reconstruct slices through the object to produce a tomogram (Japanese Patent Laid-Open No. 201253/1990).
As described above, various apparatuses and methods have been proposed with a view to improving resolution in x-ray imaging using a one- or two-dimensional detector. However, in order to apply those techniques to measurement of powder x-ray diffraction data, the following problems must be solved.
(1) Synchrotron radiation from an accelerator is x-rays which are unstable in incident strength and subject to variations. Since measurement of x-ray diffraction data using such unstable x-rays involves variations in the intensity of measurement, the prior art techniques are not capable of efficient data synthesis and fail to function properly.
(2) If highly directional parallel-beam x-rays such as synchrotron radiation are employed to measure ultrahigh resolution and complexly shaped diffraction data, the prior art involving the correction of shape and detection efficiency of sensing elements (Japanese Patent Laid-Open No. 51881/1999) is so strongly correlated to the problem of 1) that consistency in data correction is not guaranteed.
An object, therefore, of the present invention is to improve the angular resolution in the measurement of x-ray powder diffraction data with a one- or two-dimensional detector to thereby ensure that the aforementioned problems with employing parallel-beam x-rays such as synchrotron radiation that inherently undergo attenuation of incident strength over time and which are highly directional are solved while permitting powder diffraction data of high angular resolution to be measured in high precision using a one- or two-dimensional detector.
In the conventional methods of measuring powder x-ray diffraction data, a one-dimensional detector (PSPC) or a two-dimensional detector (imaging plate, IP) remains fixed or is moved during data measurement. According to the invention, a high-resolution powder diffraction experiment is performed using synchrotron radiation or other highly directional, parallel-beam x-rays as an x-ray source in such a way that a detector mounted on a measuring instrument such as a diffractometer is finely moved by distances smaller than the distance between adjacent x-ray detection units (pixels) in order to measure data for interpolation between pixels and the obtained interpolating data are put together to thereby improve the spatial resolution in measurement that has been limited by the detection unit in the detector (i.e., the fineness of each pixel in the imaging plate). This can theoretically improve resolution without increasing the size of the measuring instrument or decreasing the size of the detector.
The idea of the present invention which relies upon detector movement for improving spatial resolution finds a lot of parallels in the prior art. What is unique about the invention is that it solved the first problem of the prior art by synthesizing final powder diffraction data with individual pieces of data being scaled on the basis of their background intensities. By this technology of the invention, variations in the intensity of synchrotron radiation as x-rays can be corrected simultaneously with the synthesis of data. In the method of the invention, scaling the intensity of measurement for two kinds of data on the basis of the averages of their background intensities is an indispensable step and particularly effective in an experiment using synchrotron radiation from an accelerator as an x-ray source which inherently experiences intensity attenuation over time.
By employing a large detector such as an imaging plate, measurement can be performed over a very wide area (20 cmxc3x9740 cm) at a time with the detection unit being set in a desired position. In addition, by using a two-dimensional detector that does not require the correction of detection efficiency, data synthesis can be accomplished without correcting the shape or detection efficiency of sensing elements. This contributes to solving the second problem of the prior art.
As for the improvement in statistical precision that can be achieved with a two-dimensional detector, data in a direction perpendicular to the direction of detector movement are integrated over a few pixels to construct one-dimensional powder diffraction data and, as a result, the statistical precision of data is improved by a degree corresponding to the number of pixels over which integration was made. When a two-dimensional detector is to be used in the invention, it is particularly noteworthy that a train of data in a direction perpendicular to the direction of data synthesis may be integrated over a few pixels to an extent that will not deteriorate the resolution of powder diffraction profiles but which is sufficient to improve the statistical precision of intensity. This technique has no parallel in the prior art.